Loss Factor
The square footage that a typical office tenant pays for is called "rentable square footage". This differs from the "usable square footage" that a typical office tenant actually uses. The difference between the two is called the "Loss Factor" a.k.a. the common area factor, load factor, add on factor, etc.
The Loss Factor is made up of all the various areas in the building that are either used or needed by tenants including lobbies, common bathrooms, cafeterias, fitness rooms, elevators, stairways, etc.
In Westchester and Fairfield Counties the typical loss factor is 18 to 20 percent. In New York City the typical loss factor is 30 to 35 percent, but can be higher.